Automatic repeat request (ARQ) is a communications protocol that provides error control in data transmissions. A receiver detects an error in a packet and automatically requests a transmitter to resend the packet. The process is repeated until the packet is error free or in some cases, the process is repeated if the error continues beyond a predetermined number of transmissions. The phrase “admission request” is also used to describe this type of protocol, and refers to the requests to resend corrupted data. Various types of messages associated with an admission request include an admission request, an admission request confirmed (ARC), an admission request reject (ARJ) or other similar bandwidth requests that confirm and/or reject messages.
In this type of system, a positive acknowledgment (ACK) can be returned when data is received correctly and a negative acknowledgment (NAK) can be returned when the error is detected. Different types of automatic repeat requests can occur, including a stop-and-wait, go-back-N, and selective-reject.
There are different types of ARQ schemes, commonly known as Type I, Type II, and Type III. In Type I, erroneous received packets are discarded and a new copy of the same packet is retransmitted and decoded separately. There is no combining of earlier and later received versions. In Type II, any erroneous received packets are not discarded, but are combined with additional retransmissions for subsequent decoding. In Type III, each transmitted packet is self-decodable without combining with previous packets.
It is also possible to reuse the redundancy of previously transmitted packets by soft-combining, co-combining and a combination of soft- and co-combining. Co-combining concatenates any received packets to generate a new code word and a decoder combines the transmissions at each retransmission instant to perform a correct decoding when the code rate depends on retransmission. In a combination of soft- and co-combining, retransmitted packets carry some symbols/bits identical to previously transmitted symbols/bits and some co-symbols/bits are different from these. Examples of ARQ systems are found in U.S. published patent application Nos. 2003/0097623; 2003/0120990; and 2003/0072286.
Automatic repeat requests are often used in high frequency radio communications, including “short wave” radio communications between about 3 and 30 MHz, and in much higher frequency radio communications in the gigahertz range. The automatic repeat request protocol is often used in multi-band tactical radio systems to provide reliable tactical communications for secure voice and data transmission and networking capability. For example some advanced high frequency/very high frequency/ultra high frequency (HF/VHF/UHF) radio systems have high speed data rates up to about 9,600 Bps (HF) or more and selectable ARQ modes for reduced on-the-air transmission time, while also providing enhanced and more secure data transmissions to improve communications. Some of these systems include a serial tone data modem that can operate over poor communication channels. For example, the growth of high frequency (HF) networking throughout the 1980s and mid-1990s required a more efficient protocol such that the limited HF spectrum would support larger networks and greater amounts of data traffic.
Third-generation communications protocols have now become established and include NATO STANAG-4538, the disclosure which is hereby incorporated by reference in its entirety. Third-generation improvements include higher data throughput, automatic link establishment (ALE), increased channel capacity, quicker link establishment, and improved signal-to-noise ratio (SNR). Various systems in a third-generation ALE include the use of linking, and use of data transfer protocol units, burst PSK waveforms, multi-channel access, and Carrier Sense Multi-Access with Collision Avoidance (CSMA/CA) channel access procedures. Any software used by radios and base stations typically connect to an Ethernet-PCP/IP local area network, in which a radio could operate as a “radio” and mail server and allow delivery of e-mail and files across multiple transmission media including any HF/VHF/UHF radio, LAN, landline, microwave, or satellite systems. Such systems could form a wireless gateway with wireless message terminals.
Many of the modems used in these systems are especially susceptible to frequency shifts and frequency-spread and multi-path distortions. Modern communication systems set a high level of bit error rate (BER) performance for these modems and other equipment. For example, transmission rates could be very high and high-level protocols could use adaptive controlling of the transmission rate and increase data transmission reliability under poor conditions. These automatic repeat request communication systems as described sometimes have high error rates in the presence of severe inter-symbol interference and channel noise and adaptive channels.